Techniques using an optical fiber sheet as a breathing sensor are disclosed in Patent Literatures 1 to 3. An optical fiber sheet disclosed in Patent Literature 1 is formed by fixing or mixing an optical fiber to/into a sheet made of cloth or the like. This optical fiber sheet captures a shape change of the optical fiber that is produced by a body motion, as a change in polarization state of light propagating through the optical fiber. It is said that, when using this optical fiber sheet, the S/N ratio is good, a slight body motion produced by breathing can be detected, turning over and breathing can be clearly distinguished, and a respiratory status in a circumstance close to a normal living environment can be observed.
Patent Literatures 2 and 3 disclose a sleep apnea sensor that enables screening of sleep apnea syndrome in a normal sleep state completely without constraint at home. This sleep apnea sensor detects a body motion by measuring a change in light quantity of transmission light based on an excessive loss produced by a lateral pressure applied on the optical fiber. This sleep apnea sensor is small and silent, can be used by simply lying on an optical fiber sheet laid under a sheet covering the bedding, and can perform a measurement with nothing on the body and with a simple operation. The optical fiber sheets disclosed in these literatures can observe a state of breathing in a circumstance close to a normal living environment.